This invention relates generally to an electrophotographic printing machine, and more particularly concerns an improved optical system for utilization therein.
In the process of electrophotographic printing, a photoconductive surface is uniformly charged. The charged photoconductive surface is irradiated with a light image of an original document. The light image projected onto the charged photoconductive surface selectively discharges the charge to record an electrostatic latent image thereon. During development, toner particles are electrostatically attracted to the latent image rendering it visible. Subsequently, the toner powder image is transferred from the photoconductive surface to a sheet of support material. The powder image is permanently affixed to the sheet of support material producing a copy of the original document thereon. The foregoing process is described in greater detail in U.S. Pat. No. 2,287,691 issued to Carlson in 1942.
Multi-color electrophotographic printing is substantially the same as the heretofore discussed process. The principle distinction resides in that the optical system forms successive filtered light images of the original document rather than a total light image. The filtered or single color light image expose the charged photoconductive surface to create a single color electrostatic latent image thereon. The single color electrostatic latent image is developed with toner particles complementary in color thereto. The resulting single color toner powder image is transferred from the electrostatic latent image to a sheet of support material. This process is repeated a plurality of cycles with differently colored light images and their respective complementarily colored toner particles. Each single color toner powder image is transferred to the sheet of support material in superimposed registration with the prior toner powder image. In this manner, a multi-layered toner powder image is produced on the sheet of support material which after being permanently affixed thereto forms a color copy.
In all of the foregoing processes, the density of toner powder images is dependent upon the difference in voltage between the development system and the electrostatic latent image. Toner particles are attracted to those areas of the photoconductive surface having a voltage thereon greater than that of the development system. The areas of the photoconductive surface which are irradiated by the light image are discharged. The degree of discharge is dependent upon the intensity of the light image impinging thereon. Hence, it is highly desirable to provide a light image which is representative of the original document rather than having errors therein due to light ray non-linearities.
It is well known that the illumination at an image point is in proportion to the Cos.sup.4 of the solid angle between the illumination point and the image point. Thus, it can be seen that the illumination on a photoconductive surface will fall off quite rapidly as the solid angle increased. Various approaches have been devised to compensate for this effect. Typically, a sheet of opaque material having a butterfly slit therein is employed. The area of the slit is inversely proportional to the illumination profile. Heretofore, the optical system employed only one exposure slit. Frequently, it is necessary to calibrate the printing machine by utilizing a plurality of exposure slits to optimize the copy produced thereby. Successive exposure slits are inserted into the machine during the calibration process. This permits determination of the optimum exposure slit. Once the optimum exposure slit is determined, it is inserted in the machine and remains there for the duration of operation or, until a subsequent calibration is required. To achieve the foregoing, a plurality of exposure slits are generally stored loosely within the machine cabinet. Occassionally, these loosely stored exposure slits are lost and must be replaced. After extended usage, the exposure slits may become dirty and may have to be replaced. In addition, the printing machine may require re-calibration after a period of time to correct for the aging of the light source. Thus, it would be highly desirable to have a plurality of readily interchangeable exposure slits integral with the optical system of the electrophotographic machine.
Accordingly, it is a primary object of the present invention to improve the optical system of an electrophotographic printing machine by incorporating therein a plurality of readily interchangeable exposure slits.